Inductive proximity switches are widely used in industrial automation technology. In contrast to switching contacts and capacitive proximity switches, inductive proximity switches are insensitive to pollution and relatively inured to electromagnetic impacts. Inductive proximity switches are subject to a plurality of implementations. For instance, small sized proximity switches can be cast in plastic screws to place them into threads provided in automation facilities for sensing metal bodies in the proximity.
In general, inductive proximity switches comprise a coil that is usually accommodated in half a ferrite basin core, and an oscillator circuit that is tuned such that it oscillates with a high-quality or high-performance coil, but does not oscillate with a coil of less quality or performance. The oscillation amplitude is rectified, and a digital signal derived therefrom is issued to an output signal. If a large metal object or body is present in the stray field of the coil, the otherwise high quality of the coil will be reduced to a small value. By means of an evaluation circuit evaluating the output signal it can be recognized whether a metal object is in the vicinity of the proximity switch or not.
Since monolithically integrated coils may have a low quality and a smaller stray field due to their dimensions, the technology of known proximity switches with a discrete coil cannot readily be transferred to monolithically integrated proximity switches. Oscillating circuits of monolithically integrated coils and capacitors have, with a reasonable chip face, resonance frequencies of several GHz. The depth of penetration of the stray field in an adjacent metal object is so small with this frequency that an inductance change may be achieved, but hardly a change in quality as described above. Thus, the inductance change may not be reliably detected by amplitude detection of an oscillator vibration.